use checkout@v2 to avoid fatal: reference is not a tree (apache#6386)

"fatal: reference is not a tree" is a known issue in actions/checkout#23 and fixed in checkout@v2, update checkout used in GitHub actions.

.github/workflows/ci-cpp.yaml

@@ -33,7 +33,7 @@ jobs:
     steps:
 
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-go-functions-style.yaml

@@ -32,7 +32,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out code into the Go module directory
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-go-functions-test.yaml

@@ -35,7 +35,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-backwards-compatibility.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-cli.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-function-state.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-messaging.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-process.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-schema.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-sql.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-standalone.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-thread.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-tiered-filesystem.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-integration-tiered-jcloud.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-license.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Set up JDK 1.8
         uses: actions/setup-java@v1

.github/workflows/ci-pulsarbot.yaml

@@ -31,7 +31,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
       - name: Execute pulsarbot command
         id:   pulsarbot
         env:

.github/workflows/ci-unit-adaptors.yml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-unit-broker-sasl.yml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-unit-broker.yml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-unit-flaky.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-unit-proxy.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs

.github/workflows/ci-unit.yaml

@@ -32,7 +32,7 @@ jobs:
 
     steps:
       - name: checkout
-        uses: actions/checkout@v1
+        uses: actions/checkout@v2
 
       - name: Check if this pull request only changes documentation
         id:   docs